Protective relaying generally involves performing one or more of the following functions in connection with a protected power or energy system: (a) monitoring the system to ascertain whether it is in a normal or abnormal state; (b) metering, which involves measuring certain electrical quantities for operational control; (c) protection, which typically involves tripping a circuit breaker in response to the detection of a short-circuit condition; and (d) alarming, which provides a warning of some impending problem. Fault determination involves measuring critical system parameters and, when a fault occurs, quickly making a rough estimate of the fault location and of certain characteristics of the fault so that the power source can be isolated from the faulted line; thereafter, the system makes a comprehensive evaluation of the nature of the fault. A fault occurs when electrical current is diverted from its normal path due to external factors.
The basic principles of protective relay operation are well known. Typically, measured voltage and current values are obtained with the aid of measuring transformers in a measuring station located adjacent to a protected line. Present-day techniques employ analog-to-digital conversion and filtering of the measured values. The filtered digital values are then processed by various equations to determine various conditions of the power line. For example, in a faulty power line, the digital values are used to determine the nature and the magnitude of the fault.
Protective relays are employed to protect multiple phase systems. Moreover, the relays generally operate on data (voltage and current samples) from the multiple phases simultaneously to determine line conditions. The commonly used equations, such as Discrete Fourier Transforms, assume that all of the data from each phase is sampled at the same time. However, where it is desirable to keep the cost of manufacturing the relays to a minimum, a single analog-to-digital conversion (ADC) circuit may be employed. This single circuit then multiplexes among the various phases. The result of multiplexing into a single ADC circuit is to skew the samples in time. The amount of skew will depend upon the speed of the ADC circuit.
Skewed samples can result in a variety of measurement inaccuracies. For example, the exact impedance of a fault will be less accurate than it would be if the samples were not skewed. The skew error can also effect costs. For example, the time and associated costs required to find and repair power line faults will be affected by the inaccurate fault impedance calculation.
It has been recognized that the skew can be corrected by taking corrective measures to shift the sampled points in time. U.S. Pat. No. 5,428,549 (Chen) describes a protective relay fault detection system that uses one such skew technique in a system that samples a multiple-phase electric transmission system with a single ADC circuit. Chen provides a fault location system for locating faults associated with one or more conductors of a power distribution system, the system having an associated protective relay at a known location along the conductors. According to Chen, such a system comprises a multiplexor for obtaining multiple samples of a plurality of phase currents and voltages, the multiplexor introducing a time-skew into at least some of the samples, and an analog-to-digital convertor for converting the samples to digital sample data. Chen derives time-skew corrected sample data from the digital sample data. Thereafter, Chen derives phasor data for the currents and voltages on the basis of the time-skew corrected data and uses that corrected data to determine a fault location.
In one preferred embodiment of the Chen invention, as best illustrated in FIG. 1, the means for deriving time-skew corrected sample data from the digital sample data comprises estimating the value of a desired sample (V.sub.i+1 (t)) from the equation: EQU V.sub.i+1 (t)=(sk/dts)V.sub.i (t+sk)+(V.sub.i+1 (t+sk))(dts-sk)/dts,
where "sk" represents the time skew between the corrected sample V.sub.i +1(t) and a time-skewed sample V.sub.i+1 (t+sk), and "dts" represents a known sampling interval between samples, e.g., V.sub.i+1 (t) and previous sample V.sub.i (t). Essentially, Chen takes two measured samples, V.sub.i (t+sk) and V.sub.i+1 (t+sk), and uses the two measured skewed points to estimate a non-skewed value V.sub.i+1 (t). Chen accomplishes the estimate by using linear interpolation. Chen assumes that the corrected V.sub.i +1(t) is on a line 5 that bisects the points V.sub.i (t+sk) and V.sub.i+1 (t+sk). The error in Chen's estimate is the difference between the lines 6 and 5 at the point V.sub.i+1 (t).
As previously noted, the skewed samples introduce inaccuracies into electrical measurement equipment, such as protective relays. Therefore, it is desirable to make the samples as accurate as possible. Of course, the desired accuracy must be balanced against the cost to produce the relay. Accordingly, there is a need for an improved technique for correcting time skew in a protective relay having a single ADC circuit.